Security objects of this type may e.g. be bank notes, passports, credit cards, identity cards, packages or tags for valuable goods, data carriers, letterheads, etc. Such objects are often equipped with security features with the purpose to make counterfeiting difficult.
Security features based on holograms are widely known. While most of these features are using surface holograms (i.e. surface diffraction gratings having a depth in the order of only one or a few wavelengths), “volume holograms” have also been proposed for such applications, see e.g. EP 1 091 267. Volume holograms have gratings of a depth that is much larger than the wavelength of visible light, e.g. a depth of at least 10 μm, advantageously at least 50 μm. They can have high diffraction efficiencies, generate powerful images and have well-defined diffraction angles and wavelengths.
With the increasing skill of counterfeiters, a further improvement of such security objects is desired. Also, it is desired that the security objects have characteristics that makes them easy to be verified.